For test, rehabilitation and exercise machines where rotary motion of a person's musculature is involved, and gravity exerts a force on the body mass engaged in the rotary motion, it is important to counterbalance the effects of gravity so that the person can engage in the rotary motion without interference from the gravitational force. This is particularly important for trunk extension movement on a trunk extension/flexion test, rehabilitation and exercise machine where the person may not be able to overcome gravity and engage in trunk extension movement without some sort of counterbalancing of the force of gravity. Further, some individuals may have such severe trunk extension strength limitations that they cannot engage in trunk extension movement even if gravity is totally counterbalanced. Rather, such individuals require that the trunk extension/flexion machine positively move them through the trunk extension movement without their exerting any upward rotational force whatsoever.
Also, it is desirable to be able to quickly and easily counterbalance for different upper body masses on the trunk extension/flexion machine so that the proper counterbalancing is achieved for a variety of individuals.
Any counterbalance mechanism on a test, rehabilitation or exercise machine, to be truly effective, should not add any appreciable amount of inertia to that part of the machine which rotates. If an appreciable amount of inertia is added, the force which the person must exert to accelerate or decelerate that part of the machine which rotates is increased. This is undesirable, especially for persons who have limited rotational strength and may not be able to exert a sufficient rotational force to overcome the added inertia. Also, any added inertia will tend to force the person to engage in a greater range of trunk flexion motion than the person is capable, causing pain and possibly injury to the person.
Presently, counterbalancing on test, rehabilitation and exercise machines is accomplished by adding counterbalancing weights. These prior counterbalancing mechanisms add unwanted inertia to the system. Further, counterbalance mechanisms presently known to applicants on test, rehabilitation and exercise machines do not provide the ability to quickly and easily counterbalance a wide variety of body masses or provide positive force to move a person through a trunk extension or upward rotational movement.